Nov 29 (Reuters) - U.S. agriculture suppliers are stockpiling the Chinese chemicals that farmers need to kill crop pests and boost yields - before tariffs on them more than double on Jan. 1.

The additional tariffs, threatened by U.S. President Donald Trump, are part of an eight-month trade war between the United States and China affecting $250 billion worth of Chinese products and $113 billion in U.S. goods.

The duties could disrupt supply lines for U.S. companies that sell chemicals and fertilizers, part of a $28-billion U.S. farm chemical industry. The sector relies on Chinese imports for 40 percent of the ingredients and materials needed to make crop chemicals, according to consultancy Informa.

Nutrien Ltd - the biggest U.S. retailer of farm supplies - is stockpiling enough chemicals to last into the busy 2019 planting season, the company said in a statement. Nutrien is carrying $300 million more in chemical inventory than it had a year earlier.

Other distributors are doing the same, said Daren Coppock, CEO of the Agricultural Retailers Association. Those who have the means to stock up will do so, he said.

Higher tariffs on farm chemicals would deal another blow to an agricultural industry that has already seen prices for staple crops plummet because of the trade war between the worlds two largest economies. China has imposed import taxes on U.S. crops including soybeans - effectively shutting off an export worth $12 billion in 2017.

The Trump Administration imposed 10 percent tariffs starting Sept. 24 on about 5,700 Chinese exports, ranging from pork to bicycle tires. The duties are scheduled to rise to 25 percent on Jan. 1, and the potential economic damage adds urgency to a meeting expected between Trump and Chinese President Xi Jinping at the G20 summit in Argentina that starts Friday.

Trump has lately sounded more hopeful of resolving divisions with China, saying on Nov. 16 that higher tariffs may not be needed.

Small U.S. chemical-makers like Willowood USA and Albaugh, however, say they have already raised prices to account for a continued standoff.

The prospect of higher pesticide costs on top of weak crop prices is "worrisome," said Joe Ericson, president of the North Dakota Soybean Growers Association. The state's farmers depend on China to buy soybeans, but the oilseeds have piled up in elevators - storage facilities that buy from farmers - as the world's biggest buyer has instead turned to countries such as Brazil.

"We're hit probably more than anybody with the Chinese tariffs," Ericson said. "That's a big blow to our economy when the elevators can't get rid of them."

Retailers have seen farmers cutting back on chemical and fertilizer purchases in the wake of tariffs, Coppock said.

Farmers' costs for such supplies are a big part of that equation: U.S. soy farmers spent $52 per acre applying chemicals and fertilizer in 2017, or 12 percent of their total costs, according to the U.S. Department of Agriculture.

WINNERS AND LOSERS

Most chemicals formulated in China - including those containing the widely used weed-killers glyphosate, dicamba and 2,4-D - have incurred new tariffs since September, along with some chemical ingredients, said Sanjiv Rana, editor-in-chief of the crop protection news service Agrow, part of Informa.

The same products hit by tariffs in September will see even higher duties in January, said a spokesperson for the U.S. Trade Representative's office. The spokesperson said the tariffs are designed to discourage China's "market-distorting actions" and defend the United States from unfair trade practices.

The tariffs are having an uneven impact on chemical companies. Bayer AG, which makes glyphosate and dicamba pesticides in the United States, is largely unaffected, spokeswoman Christi Dixon said.

Tariffs do apply to glyphosate formulations made by Chinese companies, however. They directly benefit Bayer if the prices of the Chinese products exported to the United States rise because the firm sells glyphosate branded as Roundup at a premium over generic brands from China, said Jonas Oxgaard, analyst at Bernstein.

A 25 percent tariff may result in U.S. glyphosate prices rising by 15 or 20 percent, he said.

The tariffs would affect Syngenta, which imports Chinese products for its crop protection formulations, said spokesman Paul Minehart. The company is seeking alternate sources, he said.

BASF SE is concerned about the tariffs but still analyzing their potential impact, spokeswoman Odessa Hines said.

Nutrien, the retailer that sells to farmers, has already stockpiled chemicals including dicyandiamide (DCD), a fertilizer ingredient made almost exclusively in China.

"If the tariffs do come in, you're going to have higher costs for most of the major agri-chemicals," Chief Executive Chuck Magro said in an interview. "This will be something that ultimately affects the farmer."

PASSING ON COSTS

Since the Trump administration applied 10 percent tariffs in September, The Chemical Company, based in Rhode Island, has raised its DCD chemical price about 8 percent, said general sales manager AJ Petrarca.

With 25 percent tariffs coming, the company ordered more from Chinese manufacturers including Ningxia Jiafeng Chemicals Co and Beilite, swelling inventories by about 10 percent over normal levels, he said.